This invention relates to air separation. In particular, it relates to an air separation process and apparatus in which a liquid oxygen stream is withdrawn from a rectification column, is pressurized, and is then vaporized to form a high pressure, gaseous oxygen, product stream. Such processes are often referred to as `liquid pumping` processes.
Such a process may, for example, be used to provide high pressure oxygen for the manufacture of synthetic fuel gases or for the gasification of coal. By using a pump to pressurize liquid oxygen withdrawn from the rectification column, the use of an oxygen compressor is avoided. Since oxygen compressors are expensive and can be hazardous to operate, it is particularly desirable to avoid their use, and for this reason oxygen production processes using a liquid pump to withdraw oxygen in the liquid state from a rectification column find particular favor in commercial practice. Nonetheless, such processes involving the use of liquid oxygen pumping do have certain drawbacks. Suppose, for example, the oxygen product is required at a pressure of 50 atmospheres absolute (5 MPa). In order to effect vaporization of the liquid oxygen it is normal to pass it through a heat exchanger countercurrently to a stream of fluid taken from the incoming air or the nitrogen product of the process. It is desirable to maintain the specific enthalpy-temperature profile of the heat exchange stream in close conformity with that of the liquid oxygen stream being vaporized. As the temperature of the liquid oxygen stream rises, so its specific enthalpy increases. The rate of change in the change in specific enthalpy with temperature becomes progressively greater until a first maximum is reached. The specific enthalpy then increases sharply with temperature until a second maximum rate of change in the change of specific enthalpy with temperature is reached. The rate of change of specific enthalpy of the oxygen with temperature then becomes less marked. When the oxygen is at a pressure below its critical pressure, the two maxima occur at the same temperature and represent the start and finish of vaporization of the oxygen. When the oxygen is above its critical pressure, the two maxima occur at two different temperatures. The heat exchange stream also has a specific enthalpy-temperature profile with two maxima. In order best to "fit" the specific enthalpy-temperature profile of the oxygen stream being warmed with that of the heat exchange stream being cooled, the first or lower temperature maximum of the heat exchange stream should be at a temperature a few degrees K below that of the oxygen stream being warmed. This consideration imposes a requirement that the pressure of the heat exchange stream should be more than twice that of the pressure to which the liquid oxygen stream is raised. Accordingly, when the oxygen stream is required at a pressure of 50 atmospheres absolute (5 MPa), the heat exchange stream, if it is air or nitrogen, needs to be at a pressure of more than 100 atmospheres absolute. Conventional plate-fin heat exchangers cannot safely withstand such high pressures. Accordingly, the heat exchange between the liquid oxygen stream and the heat exchange stream is performed in a separate heat exchanger in parallel with a plate-fin heat exchanger used to cool a major portion of the incoming air to a temperature suitable for its separation by rectification. The parallel heat exchanger is typically of the "spiral-wound" kind. Such heat exchangers are able to withstand very high operating pressures, but are relatively expensive to fabricate. Moreover, to produce pressures in excess of 100 atmospheres absolute (10 MPa) it is generally necessary to use reciprocating rather than rotary compressors. Such reciprocating compressors are expensive, inefficient and prone to failure. GB-A-2 079 428 and GB-A-2 080 929 disclose complex liquid pumping processes which avoid the use of such high pressures in the heat exchange streams but which use an arrangement of two parallel heat exchangers each having a warm end operating at or close to ambient temperature and a cold end operating at cryogenic temperatures.